1NQI

crystal structure of lactose synthase, a 1:1 complex between beta1,4-galactosyltransferase and alpha-lactalbumin in the presence of GlcNAc

Replaces:  1J92

Summary for 1NQI

• Entry DOI: 10.2210/pdb1nqi/pdb
• Descriptor: ALPHA-LACTALBUMIN, BETA-1,4-GALACTOSYLTRANSFERASE, CALCIUM ION, ... (5 entities in total)
• Functional Keywords: lactose synthase, n-acetylglucosamine binding, transferase activator-transferase complex, transferase activator/transferase
• Biological source: Mus musculus (house mouse); More
• Total number of polymer chains: 4
• Total formula weight: 94257.48

Authors

Ramakrishnan, B.,Qasba, P.K. (deposition date: 2003-01-21, release date: 2003-02-04, Last modification date: 2024-10-16)

Primary citation

Ramakrishnan, B.,Qasba, P.K.
Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the beta-1,4-galactosyltransferase
J.Mol.Biol., 310:205-218, 2001

PubMed Abstract: The lactose synthase (LS) enzyme is a 1:1 complex of a catalytic component, beta1,4-galactosyltransferse (beta4Gal-T1) and a regulatory component, alpha-lactalbumin (LA), a mammary gland-specific protein. LA promotes the binding of glucose (Glc) to beta4Gal-T1, thereby altering its sugar acceptor specificity from N-acetylglucosamine (GlcNAc) to glucose, which enables LS to synthesize lactose, the major carbohydrate component of milk. The crystal structures of LS bound with various substrates were solved at 2 A resolution. These structures reveal that upon substrate binding to beta4Gal-T1, a large conformational change occurs in the region comprising residues 345 to 365. This repositions His347 in such a way that it can participate in the coordination of a metal ion, and creates a sugar and LA-binding site. At the sugar-acceptor binding site, a hydrophobic N-acetyl group-binding pocket is found, formed by residues Arg359, Phe360 and Ile363. In the Glc-bound structure, this hydrophobic pocket is absent. For the binding of Glc to LS, a reorientation of the Arg359 side-chain occurs, which blocks the hydrophobic pocket and maximizes the interactions with the Glc molecule. Thus, the role of LA is to hold Glc by hydrogen bonding with the O-1 hydroxyl group in the acceptor-binding site on beta4Gal-T1, while the N-acetyl group-binding pocket in beta4Gal-T1 adjusts to maximize the interactions with the Glc molecule. This study provides details of a structural basis for the partially ordered kinetic mechanism proposed for lactose synthase.

PubMed: 11419947
DOI: 10.1006/jmbi.2001.4757

Experimental method

X-RAY DIFFRACTION (2 Å)